PROJECT SUMMARY (FAST-TRACK APPLICATION) Alzheimer?s disease (AD) is a neurodegenerative disorder affecting over 5.4 million individuals in the United States alone. The complexity and multifactorial nature of AD pose unique challenges for the development of effective therapies. Efforts to target specific AD-related pathways have shown promise in animal studies, only to fail during human trials. There is a pressing need to identify novel therapeutic targets and develop new drug candidates for AD. Carriers of apolipoprotein (apo) E4, one of the three apoE isoforms (apoE2, apoE3, apoE4), are associated with 60?80% of all AD cases, making apoE4 the major genetic risk factor for AD. This proposal builds on four novel findings from our studies of mouse models and human induced pluripotent stem cell (hiPSC)?derived neurons expressing different apoE isoforms. First, expression of apoE4 in knock-in (KI) mice causes age- dependent and cell-autonomous impairment of GABAergic interneurons in the hilus of the hippocampus, which correlates with hippocampal network activity deficits and learning and memory impairments. Second, optogenetic inhibition of hilar GABAergic interneuron activity impairs spatial learning and memory in wildtype mice, indicating that hilar GABAergic interneuron impairment can directly cause cognitive deficits. Third, treatment with the GABAA receptor potentiator pentobarbital or transplantation of mouse inhibitory neuron progenitors into the hippocampal hilus rescues the learning and memory deficits in apoE4-KI mice. Fourth, apoE4 expression results in GABAergic interneuron death in hiPSC-derived neuronal cultures and in the hippocampal hilus in AD patients. Together, these findings strongly suggest that apoE4 causes GABAergic interneuron impairment, leading to learning and memory deficits, and represents a novel therapeutic target for AD. We recently identified two classes of small molecules capable of protecting GABAergic neurons from apoE4?s detrimental effects. This proposal aims to further develop, optimize, and validate compounds targeting apoE4-induced GABAergic interneuron impairment as a novel therapeutic approach for AD. The goals of this proposal are 1) to perform ADME and physicochemical studies of the initial compounds and establish a pharmacodynamic (PD) marker by in vivo hippocampal electrophysiological recordings in apoE4-KI mice, 2) to identify and optimize the lead small-molecule GABAergic interneuron protectors through structure-activity relationship studies as well as pharmacokinetic and PD studies, and 3) to test the efficacy of the lead small- molecule GABAergic interneuron protectors in apoE4-KI mice and hiPSC-derived neurons carrying the apoE4 allele.